Ring oscillator

Abstract

A ring oscillator circuit comprises a plurality of inverter states (301, 302, 303) connected in a series loop. Each stage has a voltage input (315, 315', 315") with an associated input capacitance and input threshold voltage, and a current output (316, 316', 316"). An active output circuit regulates the output currents so as to regulate the frequency of oscillation. A single reference circuit (307) can be used by more than one stage. The output circuit can vary the output currents to compensate for variable supply voltages. The oscillator can be used as part of a bias generator in an integrated circuit.

I claim: 
     
       1. A ring oscillator comprising, a plurality of cascaded inverting stages, each stage comprising an input circuit for detecting an output voltage of a preceding stage, characterized in that at least one inverting stage further comprises an output circuit having a current source that is controlled by said input circuit for supplying an output current to an input circuit of a succeeding stage. 
     
     
       2. A ring oscillator as claimed in claim 1, wherein all of the inverting stages in the ring oscillator are of the same form as the one inverting stage. 
     
     
       3. A ring oscillator as claimed in claim 1, wherein at least one input circuit comprises a logic circuit. 
     
     
       4. A ring oscillator as claimed in claim 3, wherein an input capacitance of the one inverting stage mainly comprises the inherent input capacitance of the logic circuit, the output current supplied to the succeeding stage being smaller in magnitude than an output current of said logic circuit. 
     
     
       5. A ring oscillator as claimed in claim 1, constructed in CMOS technology, wherein the output circuit is responsive to a supply voltage, which is also applied to an input circuit of the succeeding stage, in a manner so as to make the associated output current dependent on the supply voltage and thereby reduce the dependence of the frequency of oscillation on the supply voltage. 
     
     
       6. A ring oscillator as claimed in claim 1, comprising a reference circuit for determining a magnitude of the output current of at least one said output circuit. 
     
     
       7. A ring oscillator as claimed in claim 2 comprising a single reference circuit for determining the magnitudes of the output currents of the output circuits of all of the inverting stages. 
     
     
       8. A ring oscillator as claimed in claim 1, wherein the output circuit comprises a first current source for supplying a first current to an output of the one inverting stage at least when the voltage at the input circuit to the one inverting stage is in a first state, and a second current source for supplying a second current to the output, in the opposite direction to the first current, at least when the voltage at the input circuit to the one inverting stage is in a second state. 
     
     
       9. A ring oscillator as claimed in claim 8 wherein the first current source is enabled in either state of the voltage at the input circuit to the one inverting stage, the second current source being enabled only when the voltage at the input circuit to the one inverting stage is in said second state, the second current being greater in magnitude than the first current. 
     
     
       10. A ring oscillator as claimed in claim 9, wherein the magnitude of the second current is substantially twice that of the first current. 
     
     
       11. A ring oscillator as claimed in claim 9 wherein the output circuit comprises a first transistor connected between a first supply voltage terminal and the output of the one inverting stage, the first transistor forming the first current source and being connected to a reference input so as to determine the first current, the output circuit further comprising a second transistor also connected to the reference input and being connected to the first supply terminal and via a diode-connected third transistor to a second supply voltage terminal, the circuit further comprising a fourth transistor connected between the second supply terminal and the output of the one inverting stage, said fourth transistor comprising the second current source and being connected so as to mirror a current flowing in the third transistor, thereby defining the relative magnitudes of the first and second currents, a fifth transistor being connected between the second supply terminal and a control electrode of the fourth transistor so as to control conduction in the fourth transistor as a function of the state of the voltage at the input circuit. 
     
     
       12. A ring oscillator as claimed in claim 11 comprising a reference circuit which comprises a diode-connected sixth transistor connected between the second supply terminal and, via a resistance and a seventh transistor to the first supply terminal, the reference circuit further comprising an eighth transistor connected to the second supply terminal and via a diode-connected ninth transistor to the first supply terminal, and being connected to the sixth transistor so as to mirror a current flowing in said sixth transistor, a connection between the eighth and ninth transistors forming a reference output for connection to the reference input of the one inverting stage, the reference circuit further comprising a tenth transistor connected between a control electrode of the sixth transistor and the second supply terminal, and an eleventh transistor connected between the reference output and the first supply terminal, the seventh, tenth and eleventh transistors being connected so as to receive control signals for disabling the reference circuit. 
     
     
       13. A ring oscillator as claimed in claim 11 comprising a reference circuit which comprises: a long-tailed pair arrangement having its tail connected to the first supply terminal and having a current mirror active load circuit connected to the second supply terminal; a voltage divider connected between the supply terminals for biasing one transistor of the long-tailed pair; a series connection of a resistance connected to the first supply terminal and a sixth transistor connected to the second supply terminal and having a control electrode connected to an output of the current mirror active load circuit, a junction of the resistance and the sixth transistor being connected to a control electrode of the other transistor in the long-tailed pair, a seventh transistor having a control electrode connected to said current mirror output, said seventh transistor being connected to the second supply terminal and via a diode-connected eighth transistor to the first supply terminal, a connection between the seventh and eighth transistors forming a reference output for connection to the reference input of the one inverting stage. 
     
     
       14. A ring oscillator comprising: an odd number plurality of inverting stages connected in cascade in a loop circuit, each stage comprising an input circuit and an output circuit with the input circuit coupled to the output circuit of a preceding inverting stage of the loop circuit and responsive to an output voltage at an output terminal of the output circuit, and a reference circuit coupled to the output circuit of at least one inverting stage, said reference circuit including a controlled transistor current source coupled to the input circuit of said one inverting stage and responsive to a voltage developed in said input circuit in a manner so as to control the output circuit to supply a controlled output current to the input circuit of a succeeding inverter stage of the loop circuit thereby to regulate the oscillation frequency of said ring oscillator. 
     
     
       15. A ring oscillator as claimed in claim 14 wherein the input circuit and the output circuit of said one inverting stage and said reference circuit are coupled to a source of DC supply voltage, said reference circuit and said output circuit being responsive to variations in amplitude of said DC supply voltage in a manner so as to make the output current of said one inverting stage dependent on the amplitude of the DC supply voltage such that the oscillation frequency is relatively independent of the DC supply voltage. 
     
     
       16. A ring oscillator as claimed in claim 14 wherein at least one other inverting stage of the ring oscillator is of the same form as said one inverting stage, and wherein said reference circuit is coupled to said one other inverting stage so that the output circuit thereof supplies a controlled output current to the input circuit of its succeeding inverting stage of the loop circuit, the single reference circuit thereby determining output current amplitudes of the output circuits of said one inverting stage and said at least one other inverting stage. 
     
     
       17. A ring oscillator as claimed in claim 14 wherein said output circuit of the one inverting stage comprises first and second current sources for supplying to said output terminal first and second opposed currents as a function of a voltage developed in the input circuit of said one inverting stage. 
     
     
       18. A ring oscillator as claimed in claim 17 wherein said first current source supplies a first current to said output terminal independent of the voltage in said input circuit and said second current source supplies a second current to said output terminal that is determined substantially by the amplitude of said input circuit voltage.